Change blocked id cache to static.

This "shares" the cache among multiple instances of call log
fragment. This allows "clearing" the cache to propagate changes
across all instances. This should also have the side benefit
of fewer lookups.

~ Change to use ConcurrentHashMap, since it can now be accessed by
multiple call log fragments.
~ Do a little footwork with conversion, since ConcurrentHashMap
does not accept null values.

Bug: 25369105
Change-Id: Id21d1f6a18017c8e0bca88e57f09c9b4b173b843